Plain English: "trix" are for women
I like to use the word testator and I even use testatrix once in a while for fun. Everyone knows what it means and my clients like it fine.
I agree with your use of the -trix suffixes. Why abandon this ancient tradition? For the woman who handles the estate, then, do you use executrix or administratrix?
Heaven forbid a client ever got involved in something as sordid as a criminal case, but then you could refer to the female district attorney at the prosecutrix.
And my female colleagues are always appreciative when I refer to myself as a lecturer, but to one of them as a lecturix.